Thin film transistor, electronic device having the same, and method for manufacturing the same

ABSTRACT

An object of the present invention is to provide a method for manufacturing a thin film transistor which enables heat treatment aimed at improving characteristics of a gate insulating film such as lowering of an interface level or reduction in a fixed charge without causing a problem of misalignment in patterning due to expansion or shrinkage of glass. A method for manufacturing a thin film transistor of the present invention comprises the steps of heat-treating in a state where at least a gate insulating film is formed over a semiconductor film on which element isolation is not performed, simultaneously isolating the gate insulating film and the semiconductor film into an element structure, forming an insulating film covering a side face of an exposed semiconductor film, thereby preventing a short-circuit between the semiconductor film and a gate electrode. Expansion or shrinkage of a glass substrate during the heat treatment can be prevented from affecting misalignment in patterning since the gate insulating film and the semiconductor film are simultaneously processed into element shapes after the heat treatment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor element typified by afield effect transistor to be formed over a substrate having a lowstrain point and to a method for manufacturing the same, and relates toa semiconductor integrated circuit including the semiconductor elementand to a method for manufacturing the same. Specifically, the presentinvention relates to a thin film transistor in which a gate insulatingfilm is heat-treated at a temperature beyond a strain point of asubstrate such as glass and to a method for manufacturing the same.

2. Description of Related Art

In recent years, development of a system-on-panel incorporating a logiccircuit such as a CPU or a memory as well as a pixel or a driver circuitover a light transmitting insulating substrate such as glass or quartzhas been attracting attention. High-speed operation is required for adriver circuit and a logic circuit, and manufacturing a thin filmtransistor (hereinafter, also referred to as a TFT) having highswitching speed is required in order to realize it. It is effective forrealizing a TFT having higher switching speed to use a semiconductorfilm with fewer crystal defects as an active layer; to make a gateinsulating film thinner, and to miniaturize a transistor size typifiedby miniaturization of a gate length.

Characteristics required for a gate insulating film can be given asfollows: few defects in a thin film; without a fixed charge; a lowinterface level with a semiconductor film; low leakage current; and thelike. However, a gate leakage current tends to increase with a decreasein a film thickness of a gate insulating film. In addition, such a finegate insulating film that can lower a gate leakage current is requiredin order to make the gate insulating film thinner. A field effectsemiconductor device that can be driven with a low voltage and respondswell to a high drive frequency can be obtained by making the gateinsulating film thinner.

-   -   Patent Document 1: Japanese Patent Laid-Open No. H6-188421

SUMMARY OF THE INVENTION

In the case of forming a silicon film over a transparent insulatingsubstrate such as glass and manufacturing an integrated circuit by usingthe silicon film, a manufacturing technique developed in a large-scaleintegrated circuit using a single crystal silicon substrate cannot bediverted directly. This is because a processing temperature isrestricted in terms of heat resistance of glass or the like that is asubstrate over which an integrated circuit is formed as well as becauseof a crystallinity problem of a silicon film (polycrystalline siliconfilm or the like) for manufacturing an integrated circuit.

A gate insulating film which is fine and has good electrical adequacycan be formed by a CVD method, but a film formation temperature isrequired to be equal to or more than 750° C. A plasma CVD method makesit possible to form a film at a low temperature; however, it is aproblem that a film is damaged by a charged particle in plasma and adefect or a pinhole is easily caused. Further, in the case that a filmformation temperature is equal to or less than 500° C., hydrogen isincluded within a film and film stability is decreased. On the contrary,a radio frequency sputtering method can form a thin film withouthydrogen contamination. However, a film fine enough to be generally usedas a gate insulating film is not obtained by a radio frequencysputtering method in comparison with a CVD method.

Miniaturization of an element size is further required to manufacture aTFT having high switching speed that is essential for an element of alogic operation circuit and to obtain higher integration. A high-qualitygate insulating film is essential to be formed to achieve theminiaturization. The gate insulating film is preferably heat-treatedafter the formation in order to form a high-quality gate insulatingfilm. However, a substrate such as glass that expands or shrinks beforeor after applying a temperature above a strain point has a problem thatmisalignment occurs in patterning a film formed over the substrate.Therefore, it is difficult to heat-treat a gate insulating film at atemperature above a strain point of the substrate.

A typical step of manufacturing a TFT over a glass substrate isdescribed with reference to FIG. 7. FIGS. 7(E) to 7(H) are top views,and FIGS. 7(A) to 7(D) are cross-sectional views along broken lines A-Band broken lines B-C in the respective top views. In FIG. 7, steps offrom forming a semiconductor film, element isolation to manufacturing agate electrode are particularly described.

First, a base film 11 and a semiconductor film 12 are formed over aninsulating substrate 10 (FIGS. 7(A) and 7(E)). Subsequently, elementisolation is performed by processing the semiconductor film 12 intoisland shapes to form a transistor formation region 13 and a transistorformation region 14 (FIGS. 7(B) and 7(F)). Subsequently, a gateinsulating film 15 and a conductive film 16 are formed (FIGS. 7(C) and7(G)). Lastly, the conductive film 16 is patterned to form a gateelectrode 18 (FIGS. 7(D) and 7(H)). Note that a region of the gateinsulating film 15 which is not overlapped with the gate electrode 18 isetched by etching in forming the gate electrode 18, and it becomes agate insulating film 17.

As described above, element isolation is performed on the semiconductorfilm 12 to be island shapes; then, the gate insulating film 15 and theconductive film 16 are formed. Thereafter, the conductive film 16 ispatterned with the gate electrode 18 positioned to island-shapedsemiconductor films, that is, the transistor formation regions 13 and14; thus, a transistor is formed. In this method, an upper limit of aprocess temperature after processing the semiconductor film 12 intoisland shapes is determined by considering shrinkage of the substrate sothat a defect due to misalignment in patterning is not caused.

It is an object of the present invention to provide a thin filmtransistor which enables heat treatment aimed at improvingcharacteristics of a gate insulating film such as lowering of aninterface level or reduction in a fixed charge without causing a problemof misalignment in patterning due to expansion or shrinkage of asubstrate such as glass and to provide a method for manufacturing thesame.

Means to Solve the Problem

An invention of this specification is a thin film transistor comprising:an island-shaped semiconductor film and an island-shaped gate insulatingfilm patterned by using the same photomask over an insulating substrate;a side wall made of an insulating material formed on a side face of theisland-shaped semiconductor film; and a gate electrode formed over theisland-shaped gate insulating film, characterized in that the gateelectrode overlaps the side face of the island-shaped semiconductor filmwith the side wall therebetween.

An invention of this specification is a thin film transistor comprising:an island-shaped semiconductor film and an island-shaped gate insulatingfilm patterned by using the same photomask over an insulating substrate;a side wall made of an insulating material formed on side faces of theisland-shaped semiconductor film and the island-shaped gate insulatingfilm; and a gate electrode formed over the island-shaped gate insulatingfilm, characterized in that the gate electrode overlaps the side face ofthe island-shaped semiconductor film with the side wall therebetween.

An invention of this specification is a thin film transistor comprising:an island-shaped semiconductor film and an island-shaped gate insulatingfilm patterned by using the same photomask over an insulating surface;and a gate electrode formed over the island-shaped gate insulating film,characterized in that a side face of the island-shaped semiconductorfilm is insulated, and the gate electrode overlaps the insulated sideface of the island-shaped semiconductor film.

An invention of this specification is a thin film transistor comprising:an island-shaped semiconductor film and an island-shaped gate insulatingfilm patterned by using the same photomask over an insulating substrate;an insulating film patterned to cover side faces of the island-shapedsemiconductor film and the island-shaped gate insulating film and only aperipheral portion of a top face of the island-shaped gate insulatingfilm; and a gate electrode formed over the island-shaped gate insulatingfilm, characterized in that the gate electrode overlaps the side face ofthe island-shaped semiconductor film with the insulating film patternedto cover the side faces of the island-shaped semiconductor film and theisland-shaped gate insulating film and only the peripheral portion ofthe top face of the island-shaped gate insulating film therebetween.

An invention of this specification is a method for manufacturing a thinfilm transistor, characterized by comprising the steps of: forming asemiconductor film over an insulating substrate; forming a firstinsulating film over the semiconductor film; heat-treating thesemiconductor film and the first insulating film; forming anisland-shaped semiconductor film and an island-shaped gate insulatingfilm by patterning the semiconductor film and the first insulating filminto island shapes with the use of the same photomask after the heattreatment; forming a second insulating film over the island-shaped gateinsulating film; forming a side wall covering a side face of theisland-shaped semiconductor film and a side face of the island-shapedgate insulating film in a self-aligned manner by anisotropically etchingthe second insulating film; forming a conductive film over theisland-shaped gate insulating film after forming the side wall; andforming a gate electrode by patterning the conductive film.

An invention of this specification is a method for manufacturing a thinfilm transistor, characterized by comprising the steps of: forming asemiconductor film over an insulating substrate; forming an insulatingfilm over the semiconductor film; heat-treating the semiconductor filmand the insulating film; forming an island-shaped semiconductor film andan island-shaped gate insulating film by patterning the semiconductorfilm and the insulating film into island shapes with the use of oneresist mask after the heat treatment; insulating a side face of thesemiconductor film by adding oxygen or nitrogen to the side face of theisland-shaped semiconductor film without removing the resist mask;forming a conductive film over the island-shaped gate insulating film;and forming a gate electrode by patterning the conductive film.

An invention of this specification is a method for manufacturing a thinfilm transistor, characterized by comprising the steps of: forming asemiconductor film over an insulating substrate; forming a firstinsulating film over the semiconductor film; heat-treating thesemiconductor film and the first insulating film; forming anisland-shaped semiconductor film and an island-shaped gate insulatingfilm by patterning the semiconductor film and the first insulating filminto island shapes with the use of one photomask after the heattreatment; forming a second insulating film over the island-shaped gateinsulating film; patterning the second insulating film to cover edgeportions of the island-shaped semiconductor film and the island-shapedgate insulating film and only a peripheral portion of a top face of theisland-shaped gate insulating film; forming a conductive film over theisland-shaped gate insulating film; and forming a gate electrode bypatterning the conductive film.

An invention of this specification is a method for manufacturing a thinfilm transistor, characterized by comprising the steps of: forming asemiconductor film over an insulating substrate; forming a firstinsulating film on the semiconductor film; forming a first conductivefilm over the first insulating film; heat-treating the semiconductorfilm, the first insulating film, and the first conductive film; formingan island-shaped semiconductor film, an island-shaped gate insulatingfilm, and a first island-shaped conductive film by patterning thesemiconductor film, the first insulating film, and the first conductivefilm into island shapes with the use of the same photomask after theheat treatment; forming a second insulating film over the firstisland-shaped conductive film; forming a side wall covering a side faceof the island-shaped semiconductor film, a side face of theisland-shaped gate insulating film, and a side face of the firstisland-shaped conductive film in a self-aligned manner byanisotropically etching the second insulating film; forming a secondconductive film over the first island-shaped conductive film afterforming the side wall; and forming a gate electrode by patterning thefirst island-shaped conductive film and the second conductive film.

An invention of this specification is a method for manufacturing a thinfilm transistor, characterized by comprising the steps of: forming asemiconductor film over an insulating substrate; forming an insulatingfilm over the semiconductor film; forming a first conductive film overthe insulating film; heat-treating the semiconductor film, theinsulating film, and the first conductive film; forming an island-shapedsemiconductor film, an island-shaped gate insulating film, and a firstisland-shaped conductive film by patterning the semiconductor film, theinsulating film, and the first conductive film into island shapes withthe use of the same resist mask after the heat treatment; insulating aside face of the semiconductor film by adding oxygen or nitrogen to theside face of the island-shaped semiconductor film without removing theresist mask; forming a second conductive film over the firstisland-shaped conductive film; and forming a gate electrode bypatterning the first island-shaped conductive film and the secondconductive film.

An invention of this specification is a method for manufacturing a thinfilm transistor, characterized by comprising the steps of: forming asemiconductor film over an insulating substrate; forming a firstinsulating film over the semiconductor film; forming a first conductivefilm over the insulating film; heat-treating the semiconductor film, thefirst insulating film, and the first conductive film; forming anisland-shaped semiconductor film, an island-shaped gate insulating film,and a first island-shaped conductive film by patterning thesemiconductor film, the first insulating film, and the first conductivefilm into island shapes with the use of the same photomask after theheat treatment; forming a second insulating film over the firstisland-shaped conductive film; patterning the second insulating film tocover edge portions of the island-shaped semiconductor film, theisland-shaped gate insulating film, and the first island-shapedconductive film and only a peripheral portion of a top face of the firstisland-shaped conductive film; forming a second conductive film over theisland-shaped gate insulating film; and forming a gate electrode bypatterning the first conductive film and the second conductive film.

A method for manufacturing a thin film transistor according to theinvention of this specification further is characterized by comprisingthe steps of: heat-treating in a state where at least a gate insulatingfilm is formed over a semiconductor film on which element isolation isnot performed; isolating the gate insulating film and the semiconductorfilm into an element structure by using one photomask after the heattreatment; forming an insulating film covering a side face of an exposedsemiconductor film; and forming a gate electrode over the gateinsulating film. Expansion or shrinkage of a substrate such as glassduring the heat treatment can be prevented from affecting misalignmentin patterning since the gate insulating film and the semiconductor filmare simultaneously patterned and processed into element shapes after theheat treatment. The side face of the semiconductor film is exposed in acondition that the gate insulating film and the semiconductor film aresimultaneously patterned and processed into element shapes. Then, onefeature is that an insulating film covering the side face of thesemiconductor film is formed before forming an electrode such as a gateelectrode or a wiring over the gate insulating film. Thus, a shortcircuit is prevented between the semiconductor film that is processedinto an element structure and an electrode or a wiring to be formed overthe gate insulating film.

In the invention of this specification, a substrate having a lowerstrain point than a heat treatment temperature of from 600° C. to 800°C. to be applied to a gate insulating film is effectively used as aninsulating substrate over which a thin film transistor is formed,regardless of its type.

Further, a laminated film of a semiconductor film and a gate insulatingfilm on which element isolation is not performed is simultaneouslyheat-treated in the invention of this specification. Furnace or RTA(Rapid Thermal Anneal) may be used for the heat treatment. Either gasheating or lamp heating can be used in RTA treatment. Preferably, lampheating treatment may be performed with up to a conductive film forforming at least one part of a gate electrode formed over the laminatedfilm. In the case of using a halogen lamp having a peak of an emittedspectrum in an infrared region, the conductive film effectively absorbsemitted light. Not only can the gate insulating film be effectivelyheated, but also an interface between the gate insulating film and theconductive film can be heat-treated. Consequently, characteristics suchas reduction in a leakage current resulting from the interface betweenthe gate electrode and the gate insulating film can be improved.

A side face of the semiconductor film is exposed in the case ofsimultaneously performing element isolation on the laminated filmincluding the semiconductor film and the gate insulating film.Therefore, the side face of the semiconductor film is short-circuitedwith the gate electrode in the case of successively forming a conductivefilm for forming the gate electrode. Particularly, the side face of thesemiconductor film and a portion for leading the gate electrode outsidethe semiconductor film on which element isolation is performed areshort-circuited. Then, an insulating film covering the side face of thesemiconductor film is required. The insulating film covering the sideface of the semiconductor film can be formed by forming an insulatingfilm covering an entire surface of the substrate over patternedsemiconductor film and gate insulating film, anisotropically etching theinsulating film, and processing into a side wall shape in a self-alignedmanner. In addition, a method for insulating the side face of thesemiconductor film at a low temperature or a method for patterning theinsulating film to cover side faces of the semiconductor film and thegate insulating film and only a peripheral portion of a top face of thegate insulating film is given as another method for forming theinsulating film covering the side face of the semiconductor film. Theinsulating film covering the side face of the semiconductor film can beformed to have higher precision since there is no misalignment in thecase of forming in a self-aligned manner. Therefore, in the case ofintending integration, it is preferable to manufacture the insulatingfilm by a method for forming into a side wall shape or a method forinsulating the side face of the semiconductor film at a low temperature.In this way, the insulating film is formed only on a desired side faceof a semiconductor film.

According to the present invention having the above structures, a gateinsulating film can be heat-treated without a problem of an alignmentdefect in patterning even at a temperature of 700° C. thatconventionally causes a problem of misalignment in patterning due toshrinkage of a substrate such as glass.

In the present invention, a gate insulating film can be heat-treated ata temperature of 700° C. above a strain point of a substrate such asglass. Therefore, an interface level is lowered; a fixed charge isreduced; a gate leakage current is lowered; field-effect mobility,subthreshold coefficient, and the like become favorable; a change oftransistor characteristics over time during continuous operation can bereduced; a yield is improved; and variation in the characteristics isreduced, in a thin film transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1H are diagrams showing a step of manufacturing a thin filmtransistor of the present invention.

FIGS. 2A-2J are diagrams showing a step of manufacturing a thin filmtransistor of the present invention.

FIGS. 3A-D3 are diagrams showing a step of manufacturing a thin filmtransistor of the present invention.

FIGS. 4A-4C are diagrams showing a pixel structure of a display panelaccording to the present invention.

FIGS. 5A-5D are diagrams showing a structure of a display panelaccording to the present invention.

FIGS. 6A-6H are diagrams showing structures of electronic apparatusesaccording to the present invention.

FIGS. 7A-7H are diagrams showing a step of manufacturing a thin filmtransistor in which element isolation is performed on a semiconductorfilm before forming a gate insulating film.

EMBODIMENT OF THE INVENTION

Embodiment Mode 1

A glass substrate made of a material such as barium borosilicate glass,alumino borosilicate glass, or aluminosilicate glass, or the like can begiven as a substrate which can be applied in this embodiment mode.Typically, a 1737 glass substrate (strain point: 667° C.) manufacturedby Corning, Inc., AN100 (strain point: 670° C.) manufactured by AsahiGlass Co., Ltd., or the like can be applied, but there is no particularlimitation on other similar substrates.

A first inorganic insulating layer 21 made of an insulating film such asa silicon oxide film, a silicon nitride film, or a silicon oxynitridefilm (SiOxNy) is formed over a glass substrate 20, as shown in FIGS.1(A) and 1(E), using the above substrate. A typical example of the firstinorganic insulating layer 21 has a two-layer structure, which is astructure where a first silicon oxynitride film formed to be 50 nm inthickness by a plasma CVD method using SiH₄, NH₃, and N₂O as a reactivegas and a second silicon oxynitride film formed to be 100 nm inthickness by a plasma CVD method using SiH₄ and N₂O as a reactive gasare laminated.

A crystalline semiconductor film 22 serving as an active layer of a TFTis obtained by crystallizing an amorphous semiconductor film formed overthe first inorganic insulating layer 21. A crystalline silicon film orthe like can be used for the crystalline semiconductor film 22.Thickness of the amorphous semiconductor film is selected in the rangewhere thickness of the crystalline semiconductor film 22 obtained bycrystallizing the amorphous semiconductor film is to be from 20 nm to 60nm. An upper limit of the film thickness of the crystallinesemiconductor film 22 serving as an active layer of a TFT is a maximumvalue for operating as a fully depleted type in a channel region of aTFT. A lower limit of the film thickness is a limitation on a process,and is set as a minimum value required in selectively processing onlythe crystalline semiconductor film 22 during an etching step of thecrystalline semiconductor film 22.

A gate insulating film 23 is formed over the crystalline semiconductorfilm 22. A silicon oxide film formed by a reactive sputtering methodusing Ar and 02 and utilizing a Si target, a silicon oxynitride filmformed by a CVD method using SiH₄, NH₃, and N₂O as a reactive gas, orthe like can be used for the gate insulating film 23. The gateinsulating film 23 is not limited to a silicon compound, and highdielectric constant metal oxide that has a higher dielectric constantthan that of silicon oxide and by which an effect of making the gateinsulating film thinner is effectively obtained may be used. Effectivefilm thickness can be expressed as a product t·k₁/k₂ of actual filmthickness t and ratio k₁/k₂ of a relative dielectric constant k₁ of afilm material to be a benchmark such as silicon oxide to a relativedielectric constant k₂ of an actual film material. Note that a filmthickness of the gate insulating film 23 is set by a scaling law and aprocess margin, and the thickness of the gate insulating film 23 is setto be from 20 nm to 80 nm in order to manufacture a TFT with a gatelength of from 0.35 μm to 2.5 μm here.

Subsequently, a first conductive film 24 is formed over the gateinsulating film 23. A tantalum nitride film is formed by reactivesputtering using Ar and N₂ gas and utilizing a Ta target to have a filmthickness of from 10 nm to 50 nm for the first conductive film 24.Another conductive film as well as a tantalum compound may be used forthe first conductive film 24. However, the first conductive film 24 ispreferably a material that absorbs light within a wavelength ofapproximately 1 μm and a material that can have an enough selectiveratio in etching with a second conductive film 34 to be formed later.

Subsequently, the crystalline semiconductor film 22, the gate insulatingfilm 23, and the first conductive film 24 are heat-treated, as shown inFIGS. 1(B) and 1(F). RTA treatment that is capable of heating andcooling instantaneously is employed as heat treatment. A temperaturerises up to a temperature of from 600° C. to 800° C. in 10 seconds to120 seconds, and heat treatment is performed at a temperature of from600 □C to 800° C. for 30 seconds to 180 seconds in RTA treatment. Notethat there are a gas heating method using a heated gas and a lampheating method by emission of a lamp as the RTA treatment. The glasssubstrate 20 itself is heated by a gas in the case of the gas heatingmethod, and the gate insulating film 23 can be heat-treated. However,temperature rising efficiency is generally remarkably inefficient in thelamp heating method. This is because the glass substrate 20 itself ishard to be heated since a typical halogen lamp has a peak of an emittedspectrum at approximately 1 μm and the glass substrate 20 does notsufficiently absorb light within such a wavelength region by itself. Inthis embodiment mode, heat conduction to the gate insulating film 23occurs by using a tantalum nitride film as an absorber layer since thetantalum nitride film that is the first conductive film 24 absorbs lightwithin a wavelength of approximately 1 μm. Consequently, the gateinsulating film 23 can be efficiently heat-treated. Note that the heattreatment is performed at a temperature above the strain point of glass,and shrinkage of the glass substrate 20 is caused. However, a patterningdefect due to the shrinkage is not caused in a later step since thecrystalline semiconductor film 22 is not processed into an element shapeyet at the time of the heat treatment.

Subsequently, the crystalline semiconductor film 22, the gate insulatingfilm 23, and the first conductive film 24 are collectively etched intoisland shapes by using the same photomask, as shown in FIGS. 1(C) and1(G). For example, an ICP (Inductively Coupled Plasma) etching methodcan be applied as an etching method. A mixed gas of CF4 and C12 can beused as an etching gas in etching the first conductive film 24 made of atantalum nitride film. A CHF₃ gas can be used for etching the gateinsulating film 23 made of a silicon oxide film, and a mixed gas of CF4and O2 can be used in etching the crystalline semiconductor film 22 madeof a crystalline silicon film. Thus, a crystalline semiconductor film 25and a crystalline semiconductor film 28 which are processed into islandshapes, a gate insulating film 26 and a gate insulating film 29 whichare processed into island shapes, and a first conductive film 27 and afirst conductive film 30 which are processed into island shapes areformed.

Subsequently, an insulating film 31 covering an entire surface of theglass substrate 20 is formed to cover exposed side faces of thecrystalline semiconductor film 25 and the crystalline semiconductor film28 as shown in FIGS. 1(D) and 1(H). A silicon oxide film formed by a lowpressure CVD method which isotropically grows to have a film thicknessof from 500 nm to 1.5 μm is used as the insulating film 31. Note thatthe insulating film 31 is only necessary to be an insulating film, and asilicon nitride film or a silicon oxynitride film can be used withoutbeing limited to the silicon oxide film.

Subsequently, a side wall 32 and a side wall 33 covering side faces ofthe crystalline semiconductor film 25 and the crystalline semiconductorfilm 28 and side faces of the gate insulating film 26 and the gateinsulating film 29 can be formed as shown in FIGS. 2(A) and 2(F) byapplying a predetermined bias voltage to a glass substrate 20 side andanisotropically etching the insulating film 31 made of a silicon oxidefilm. Effective thickness of the side wall 32 and the side wall 33 in aportion covering the side faces of the crystalline semiconductor film 25and the crystalline semiconductor film 28 in a direction perpendicularto the side faces is set equal to or thicker than effective thickness ofthe gate insulating film 26 and the gate insulating film 29. Forinstance, when all the gate insulating film 26, the gate insulating film29, the side wall 32, and the side wall 33 are made of silicon oxidefilms, thickness of the side wall 32 and the side wall 33 in a portioncovering the side faces of the crystalline semiconductor film 25 and thecrystalline semiconductor film 28 in a direction perpendicular to theside faces is set at equal to or more than from 20 nm to 80 nm that isthickness of the gate insulating film 26 and the gate insulating film29. In this way, a short circuit and a current leakage can be suppressedbetween a portion for leading a gate electrode outside the semiconductorfilm on which element isolation is performed and the side faces of thecrystalline semiconductor film 25 and the crystalline semiconductor film28.

Then, a second conductive film 34 shown in FIGS. 2(B) and 2(G) isformed. A tungsten film having a film thickness of from 300 nm to 500 nmis used as the second conductive film 34 in this embodiment mode. Thesecond conductive film 34 is not limited to a tungsten film, and is onlynecessary to be a conductive film. However, a material having an enoughselective ratio in etching with the first conductive film 24 ispreferably used for the second conductive film 34.

A first conductive layer 37 and a first conductive layer 40 made oftantalum nitride and a second conductive layer 38 made of tungsten,which are processed into a shape of a gate electrode, are obtained byetching the first conductive film 24 and the second conductive film 34,as shown in FIGS. 2(C) and 2(H). Here, a structure in which the firstconductive layer 37, the first conductive layer 40, and the secondconductive layer 38 have different tilt angles in edge portions ismanufactured. The first conductive layer 37, the first conductive layer40, and the second conductive layer 38 having different tilt angles inthe edge portions are formed by performing two-stage etching treatmenton the first conductive film 24 and the second conductive film 34. Inthe first stage of etching, both tungsten and tantalum nitride aresimultaneously etched by applying a predetermined voltage to the glasssubstrate 20 with the use of a mixed gas of CF₄, Cl₂, and O₂ as anetching gas, and a layer made of tungsten and a layer made of tantalumnitride having the same tilt angles in edge portions are manufactured.Subsequently, in the second stage of etching, only the layer made oftungsten is anisotropically etched by applying a predetermined biasvoltage to the glass substrate 20 under the first stage etchingcondition in which the etching gas is replaced with SF₆, Cl₂, and O₂. Inthis way, the first conductive layer 37, the first conductive layer 40,and the second conductive layer 38 having different tilt angles in theedge portions are formed. Note that the gate insulating film 26, thegate insulating film 29, the side wall 32, and the side wall 33 are alsoetched to be a gate insulating film 36, a gate insulating film 39, aside wall 35 a, and a side wall 35 b respectively in a process ofetching the first conductive layer 37, the first conductive layer 40,and the second conductive layer 38.

Then, a desired quantity of impurities is doped. 41 and 44 in FIGS. 2(D)and 2(I) become a source or a drain doped with an n-type or a p-typeimpurity in high concentration respectively; 42 and 45 become dopingregions doped with an n-type impurity in lower concentration than thosein the source or drain 41 and the source or drain 44 (Gate OverlappedLightly Doped Drain) since they are doped through the edge portions ofthe first conductive layer 37 and the first conductive layer 40 that areparts of the gate electrodes; and 43 and 46 become channel regions.

Thereafter, as shown in FIGS. 2(E) and 2(J), a silicon oxynitride filmcontaining hydrogen is formed as an insulating film 51 to have a filmthickness of 100 nm by a plasma CVD method, and the crystallinesemiconductor film 25, the crystalline semiconductor film 28, the gateinsulating film 36, and the gate insulating film 39 are hydrogenated byheat-treating at 410° C. Further, a silicon oxide film is formed as aninterlayer insulating film 52 to have a film thickness of from 400 nm to600 nm by a CVD method. Note that phosphorous glass (PSG), boronphosphorous glass (BSG), or phosphorous boron glass (PBSG) can beapplied to the interlayer insulating film 52. A porous film or a lowdielectric constant film such as acrylic of an organic resin system orTeflon (registered trademark) can be used as the interlayer insulatingfilm 52 as well. Then, a silicon nitride film is formed as a barrierfilm 53 to have a film thickness of 100 nm by a sputtering method. Inthe next place, a wiring 47, a wiring 48, a wiring 49, and a wiring 50are formed after forming contact portions to reach the source or drain41 and the source or drain 44 by etching the barrier film 53, theinterlayer insulating film 52, the insulating film 51, the gateinsulating film 36, and the gate insulating film 39. A laminatestructure of a titanium film with a thickness of 60 nm, a titaniumnitride film with a thickness of 40 nm, an aluminum film with athickness of 300 nm, and a titanium film with a thickness of 100 nm isused for the wirings 47 to 50. However, a structure of the wirings 47 to50 is not limited to the above structure, and copper can be used inplace of aluminum. A film in contact with the aluminum film is notlimited to titanium nitride, and tantalum nitride, tungsten nitride, orthe like can be used in the wirings 47 to 50.

Embodiment Mode 2

Ozone is used at a temperature of 500° C. to oxidize the crystallinesemiconductor film 25, the crystalline semiconductor film 28, the gateinsulating film 26, the gate insulating film 29, the first islandconductive film 27, and the first island conductive film 30 which areprocessed into island shapes as shown in FIGS. 1(C) and 1(G) inEmbodiment Mode 1. Thus, oxide films are formed on exposed side faces ofthe crystalline semiconductor film 25 and the crystalline semiconductorfilm 28, and effective thickness of the oxide films is set equal to orthicker than effective thickness of the gate insulating film 26 and thegate insulating film 29 as shown in FIGS. 3(A) and 3(C), therebypreventing a short circuit between a gate electrode to be formed laterand the side faces of the crystalline semiconductor film 25 and thecrystalline semiconductor film 28. Note that an oxide film, a nitridefilm, an oxynitride film, or the like can be used as an insulating filmto be formed on the side faces of the crystalline semiconductor film 25and the crystalline semiconductor film 28. Plasma oxidization as well asa method using an ozone gas can be performed by using plasma includingoxygen as an oxidation method. In addition, washing by using ozone watermay be performed as an oxidation method, and in this case, oxidation canbe performed efficiently by irradiating a surface of the glass substrate20 with ultraviolet light. Plasma nitriding can be performed by usingplasma including a nitrogen gas as a nitriding method. Further, only theside faces of the crystalline semiconductor film 25 and the crystallinesemiconductor film 28 can be selectively made insulative by dopingoxygen or nitrogen with a resist mask used in patterning theisland-shaped crystalline semiconductor film 25, the island-shapedcrystalline semiconductor film 28, the gate insulating film 26, the gateinsulating film 29, the first conductive film 27, and the firstconductive film 30 remained.

Embodiment Mode 3

An insulating film is formed over an entire surface of the glasssubstrate 20 to cover the crystalline semiconductor film 25, thecrystalline semiconductor film 28, the gate insulating film 26, and thegate insulating film 29 after forming the crystalline semiconductor film25, the crystalline semiconductor film 28, the gate insulating film 26,the gate insulating film 29, the first conductive film 27, and the firstconductive film 30 which are processed into island shapes as shown inFIGS. 1(C) and 1(G) in Embodiment Mode 1. A silicon oxide film formed tohave a thickness of from 50 nm to 100 nm by a CVD method is used as theinsulating film. The insulating film is not limited to the silicon oxidefilm formed by a CVD method, and a silicon nitride film, a siliconoxynitride film, or the like can be used. A film formation method isalso not limited to a CVD method, and a sputtering method, or the likecan be applied. Then, the insulating film is patterned to forminsulating layers 54 to 57 as shown in FIGS. 3(B) and 3(D). Theinsulating layers 54 to 57 are shaped to cover side faces of theisland-shaped crystalline semiconductor film 25 and the island-shapedcrystalline semiconductor film 28 in a region overlapped with at least agate electrode to be formed later, and effective thickness of theinsulating layers 54 to 57 is set equal to or thicker than effectivethickness of the gate insulating film 26 and the gate insulating film29, thereby preventing a short circuit between the crystallinesemiconductor film 25 and the crystalline semiconductor film 28 and thegate electrode to be formed later.

Embodiment

Embodiment 1

A cross-sectional structure in the case of manufacturing a displaydevice by using a typical thin film transistor manufactured according toEmbodiment Modes 1 to 3 is described.

A TFT disposed in a driver circuit portion and a pixel portion is formedover a substrate 500 having an insulating surface according tomanufacturing steps described in the above embodiment modes. Thereafter(FIG. 4(A)), a first electrode 501 made of a transparent conductive filmis formed to electrically connect with a wiring 507 of a driving TFT513. The transparent conductive film is preferably made of a materialhaving a high work function, and the following can be given as anexample thereof: a compound of indium oxide and tin oxide (ITO); acompound of indium oxide and zinc oxide; zinc oxide; tin oxide; indiumoxide; titanium nitride; or the like. In this embodiment, an ITO filmwith a thickness of 0.1 μm was formed by a sputtering method as thefirst electrode 501.

In this embodiment, a method for forming the transparent conductive filmto electrically connect with the wiring 507 after forming the wiring 507was described, but the transparent conductive film may be formed byanother method. For example, the wiring 507 of the TFT may be formed toelectrically connect with the first electrode after forming thetransparent conductive film and forming the first electrode bypatterning the transparent conductive film. In addition, after formingthe wiring 507 of the TFT, an insulating film is formed over the wiring507, and thereafter, a contact hole is formed in the insulating film toreach the wiring 507. Then, the transparent conductive film may beformed to electrically connect with the wiring 507 through the contacthole.

Subsequently, an insulating film 504 is formed to cover an end face ofthe first electrode 501. There is no particular limitation on a materialfor forming the insulating film 504, and the insulating film 504 can bemade of an inorganic or organic material. The insulating film 504 ispreferably made of a photosensitive organic material since a shape ofthe opening portion provided for the insulating film 504 becomes such ashape that disconnection in a light emitting layer to be evaporated overthe insulating film 504 is hardly caused. Namely, the shape of theopening portion provided for the insulating film 504 can be made intosuch a gently curved shape that a slope of a surface on which the lightemitting layer is formed continuously changes, thereby improvingcoverage of the light emitting layer and preventing disconnection in thelight emitting layer. Consequently, a short circuit between an anode anda cathode due to breaking of a wiring of a light emitting element isreduced. In addition, the light emitting layer can be prevented frombecoming thin partly and an electric field can be prevented fromconcentrating locally in the light emitting layer. A photosensitivepolyimide resin, photosensitive acrylic, or the like can be used as thephotosensitive organic material for forming the insulating film 504. Forexample, in the case of using a negative photosensitive resin as amaterial of the insulating film 504, a shape of an upper end portion ofthe insulating film 504 in contact with a top face of the firstelectrode 501 can be formed to be a curved shape that has a center ofcurvature below a tangent to a top face of the insulating film 504 andthe upper end portion of the insulating film 504 and is determined by afirst curvature radius. A shape of a lower end portion of the insulatingfilm 504 can be formed to be a curved shape that has a center ofcurvature above a tangent to the first electrode 501 and the lower endportion of the insulating film 504 and is determined by a secondcurvature radius. The first and the second curvature radii arepreferable from 0.2 μm to 3 μm, and an angle of a side wall of theopening portion to the first electrode 501 is preferably equal to ormore than 35°.

Subsequently, dust or the like is removed by wiping with a porous bodyof a PVA (polyvinyl alcohol) system. In this embodiment, fine powder(dust) generated in etching the first electrode 501 made of an ITO orthe insulating film 504 was removed by wiping with the porous body ofPVA.

Subsequently, a light emitting layer 502 is formed to be in contact withthe first electrode 501. The light emitting layer 502 is formed by anevaporation method or an application method (a spin coating method, anink-jetting method, or the like). In this embodiment, a method forevaporating with an evaporation source moving was employed. In thismethod, an organic compound which is a material of the light emittinglayer 502 and is put in the evaporation source is vaporized in advanceby resistance heating, and a shutter is provided to prevent thevaporized organic compound from being scattered in a direction of theglass substrate 20 from the evaporation source. In evaporating, thevaporized organic compound was scattered upwardly by opening the shutterand was evaporated over the glass substrate 20 through an openingportion provided for a metal mask, thereby forming the light emittinglayer 502.

Note that PEDOT may be entirely applied and may be baked as treatmentbefore evaporation of the light emitting layer 502. It is preferable towash PEDOT after PEDOT is once applied and to apply PEDOT again sincePEDOT has poor wettability with ITO that is the first electrode 501. Inthis way, after applying PEDOT, a heat treatment is performed at normalpressure to vaporize moisture, and then, a heat treatment is performedunder reduced pressure.

One of or a plurality of layers to be provided between the firstelectrode and a second electrode forming a light emitting element isgenerically referred to as the light emitting layer (layer including alight emitting material) 502. The light emitting layer 502 can be formedby using a low molecular weight organic compound material, a highmolecular weight organic compound material, or a mixture thereofappropriately. Further, a mixed layer in which an electron transportingmaterial and a hole transporting material are appropriately mixed, or amixed bonding in which a mixed region is formed at a bond interface ofeach material may be formed. In addition to an organic material, aninorganic light emitting material may be used. Further, a laminatestructure of the light emitting layer 502 is not particularly limited,and a structure in which layers made of a low molecular weight materialare laminated or a structure in which a layer made of a high molecularweight material and a layer made of a low molecular weight material arelaminated may be adopted.

Subsequently, a second electrode 503 is formed over the light emittinglayer 502. The second electrode 503 is made of a laminated film of athin film containing metal having a small work function (Li, Mg, or Cs)and a transparent conductive film laminated over the thin filmcontaining Li, Mg, or the like. The film thickness is properly set tofunction as a cathode, but here, it is set at approximately from 0.01 μmto 1 μm in thickness by a known method (an electron beam evaporationmethod or the like). However, in the case of employing an electron beamevaporation method, radiation is generated when an acceleration voltageis too high, and thus, a TFT is damaged. However, when an accelerationvoltage is too low, film formation speed is slowed down and productivitydecreases. Therefore, the second electrode 503 is formed so as not to beexcessively thicker than such a film thickness that the second electrodefunctions as a cathode. When the second electrode 503 is thin, theproductivity is not affected significantly even if the film formationspeed is slow. However, a problem of increase in resistance arises whenthe cathode is thin. The problem can be solved by forming Al or the likewhich is a low-resistance metal over the cathode by resistance heatingevaporation or a sputtering method to be a laminated structure. In thisembodiment, Al—Li was formed to be 0.1 μm in thickness as the secondelectrode 503 by an electron beam evaporation method.

Subsequently, a protective film 505 is formed over the insulating film504 and the second electrode 503. A film that is hardly penetrated,compared to other insulating films, by a substance such as moisture oroxygen to be a cause of accelerating deterioration of a light emittingelement 506 is used as the protective film 505. Typically, a DLC film, acarbon nitride film, a silicon nitride film formed by an RF sputteringmethod, or the like is preferably used. In addition, film thicknessthereof is preferably approximately from 10 nm to 200 nm. In thisembodiment, a silicon nitride film was formed to have a thickness of 100nm by a sputtering method.

A laminate of the first electrode 501, the light emitting layer 502, andthe second electrode 503, which is formed in the above-described steps,corresponds to the light-emitting element 506. The first electrode 501corresponds to an anode, and the second electrode 503 corresponds to acathode. In the present invention, there are singlet excitation andtriplet excitation as an excitation state of the light emitting element506, and luminescence can be generated through either excitation state.

FIG. 4(B) shows a top view of one pixel in a display device using alight emitting element. FIG. 4(B) shows a state that up to a pixelelectrode 501 is formed. In the top view of FIG. 4(B), a cross sectionalview equivalent to A-B-C corresponds to FIG. 5(A). Further, FIG. 4(C)shows a circuit diagram of one pixel equivalent to FIG. 4(B). In FIGS.4(B) and 4(C), reference numeral 508 denotes a source line; 509, a gateline; 510, a power source line; 511, a capacitor element; 501, the firstelectrode (pixel electrode); 512, a switching TFT; and 513, the drivingTFT.

In this embodiment, a case where so-called bottom emission in whichlight emitted from the light emitting element 506 was extracted from aside of the substrate 500 was performed was described. However,so-called top emission in which light is extracted from a directionopposite to the substrate 500 may be performed, instead. In that case,the first electrode 501 is formed to correspond to the cathode, and thesecond electrode 503 is formed to correspond to the anode. Further, thesecond electrode 503 is preferably made of a transparent material. Inaddition, the driving TFT 513 is preferably made of an n-channel TFT.Note that a conductivity type of the driving TFT 513 may beappropriately changed, but the capacitor element 511 is arranged to holdvoltage between the gate and the source. Note that the case of a lightemitting device using the thin film transistor and the light emittingelement of the present invention is described in this embodiment;however, the present invention can be applied to another display devicesuch as a liquid crystal display device.

This embodiment can be freely combined with the above-describedembodiment modes.

Embodiment 2

An embodiment of the present invention is described with reference toFIG. 5. FIG. 5(A) is a top view of a display panel formed by sealing asubstrate over which a TFT is formed with a sealing material. FIG. 5(B)is a cross-sectional view along a line B-B′ in FIG. 5(A). FIGS. 5(C) and5(D) are cross-sectional views along a line A-A′ in FIG. 5(A). Note thatFIG. 5(C) is a cross-sectional view of a display panel performing bottomemission in which light is emitted in a direction of the substrate overwhich a TFT is formed. FIG. 5(D) is a cross-sectional view of a displaypanel performing top emission in which light is emitted in a directionopposite to the substrate over which a TFT is formed.

In FIGS. 5(A) to 5(D), a pixel portion (display portion) 602, a signalline driver circuit 603 which is disposed to surround the pixel portion602, scanning line driver circuit 604 a, and scanning line drivercircuit 604 b are all disposed over a substrate 601, and a seal material606 is provided to surround all of them. The structure described in theabove Embodiment 1, or the like can be applied to a structure of thepixel portion 602. A glass material, a metal material, a ceramicmaterial, or a plastic material is used as the seal material 606. Theseal material 606 may be provided to partially overlap the signal linedriver circuit 603, the scanning line driver circuit 604 a, and thescanning line driver circuit 604 b.

In a display panel shown in FIG. 5(C), a sealing material 607 isprovided by using the seal material 606 as an adhesive layer, so that aclosed space 608 is formed with the substrate 601, the seal material606, and the sealing material 607. A hygroscopic agent 609 is providedin advance for a depression of the sealing material 607, so that it hasa function of absorbing moisture, oxygen, and the like to keep anatmosphere clean in an inner portion of the closed space 608, therebysuppressing deterioration of the light emitting element. The depressionis covered with a cover material 610 with a fine mesh shape. The covermaterial 610 allows air and moisture to pass therethrough but not thehygroscopic agent 609. Note that the closed space 608 may be filled witha noble gas such as nitrogen or argon, or can be filled with a resin ora liquid as long as it is inert.

In a display panel in FIG. 5(D), a transparent opposing substrate 621 isprovided by using the seal material 606 as an adhesive layer, so that aclosed space 622 is formed with the substrate 601, the opposingsubstrate 621, and the seal material 606. The opposing substrate 621 isprovided with a color filter 620 and a protective film 623 forprotecting the color filter. Light emitted from the light emittingelement disposed in the pixel portion 602 is exteriorly emitted throughthe color filter 620, and the display panel performs multicolor display.The closed space 622 is filled with an inert resin, an inert liquid, orthe like. In the case of performing multi color display, the lightemitting layer may be set to emit each color of RGB, or a pixel providedwith a light emitting layer that emits white light may be arranged inorder that the color filter or a color conversion layer is used.

An input terminal portion 611 for transmitting a signal to the signalline driver circuit 603, the scanning line driver circuit 604 a, and thescanning line driver circuit 604 b is provided over the substrate 601. Adata signal such as a video signal is transmitted to the input terminalportion 611 through an FPC 612. A cross section of the input terminalportion 611 is as shown in FIG. 5(B), and an input wiring 613 made of awiring which is formed together with the scanning line or the signalline is electrically connected to a wiring 615 provided on a side of theFPC 612 by using a resin 617 in which a conductive material 616 isdispersed. Note that a spherical high molecular weight compound platedwith gold or silver may be used as the conductive material 616.

In this embodiment, an example of applying the present invention to thelight-emitting panel using the light emitting element is described;however, the present invention may be applied to a liquid crystal panelusing a liquid crystal display element.

This embodiment can be freely combined with other above-describedembodiment modes and embodiments.

Embodiment 3

The following can be given as examples of electronic apparatuses towhich the present invention is applied: a video camera; a digitalcamera; a goggle type display; a navigation system; an audio reproducingdevice (car audio, or the like); a laptop computer; a game machine; apersonal digital assistant (a mobile computer, a cellular phone, or thelike); an image reproducing device including a recording medium; and thelike. Practical examples of these electronic apparatuses are shown inFIG. 6.

FIG. 6(A) shows a light emitting device, which includes a chassis 2001,a supporting section 2002, a display portion 2003, speaker portions2004, a video input terminal 2005, and the like. The present inventioncan be applied to the display portion 2003. The light emitting device isself-luminous and does not need a backlight, so that the display portioncan be made thinner than that of a liquid crystal display. Note that thelight emitting device includes all display devices for displayinginformation, including ones for personal computers, for TV broadcastingreception, and for advertisement.

FIG. 6(B) shows a digital still camera, which includes a main body 2101,a display portion 2102, an image receiving portion 2103, operation keys2104, an external connection port 2105, a shutter 2106, and the like.The present invention can be applied to the display portion 2102.

FIG. 6(C) shows a laptop personal computer, which includes a main body2201, a chassis 2202, a display portion 2203, a keyboard 2204, anexternal connection port 2205, a pointing mouse 2206, and the like. Thepresent invention can be applied to the display portion 2203.

FIG. 6(D) shows a mobile computer, which includes a main body 2301, adisplay portion 2302, an electric switch 2303, operation keys 2304, aninfrared port 2305, and the like. The present invention can be appliedto the display portion 2302.

FIG. 6(E) shows a portable image reproducing device including arecording medium (specifically, a DVD reproducing device), whichincludes a main body 2401, a chassis 2402, a display portion A 2403, adisplay portion B 2404, a recording medium reading portion 2405,operation keys 2406, speaker portions 2407, and the like. The displayportion A 2403 mainly displays image information whereas the displayportion B 2404 mainly displays text information. The present inventioncan be applied to the display portion A 2403 and the display portion B2404.

FIG. 6(F) shows a goggle type display (head mounted display), whichincludes a main body 2501, display portions 2502, and arm portions 2503.The present invention can be applied to the display portions 2502.

FIG. 6(G) shows a video camera, which includes a main body 2601, adisplay portion 2602, a chassis 2603, an external connection port 2604,a remote control receiving portion 2605, an image receiving portion2606, a battery 2607, an audio input portion 2608, operation keys 2609,an eye piece portion 2610, and the like. The present invention can beapplied to the display portion 2602.

FIG. 6(H) shows a cellular phone, which includes a main body 2701, achassis 2702, a display portion 2703, an audio input portion 2704, anaudio output portion 2705, operation keys 2706, an external connectionport 2707, an antenna 2708, and the like. The present invention can beapplied to the display portion 2703. Note that if the display portion2703 displays white letters on black background, the cellular phoneconsumes less power.

As described above, the applicable range of the present invention is sowide that the invention can be applied to electronic devices of variousfields. In addition, the electronic devices of this embodiment can befreely combined with the above embodiment modes and embodiments.

Advantageous Effect of the Invention

According to the present invention, a gate insulating film can beheat-treated without an alignment defect in patterning even at atemperature of 700° C. that conventionally causes a problem of alignmentin patterning due to shrinkage of a substrate such as glass.

By heat-treating a gate insulating film at a temperature of 700° C.above a strain point of glass, an interface level is lowered; a fixedcharge is reduced; a gate leakage current is lowered; field-effectmobility, subthreshold coefficient, and the like become favorable; achange of transistor characteristics over time during continuousoperation is reduced; a yield is improved; and variation in thecharacteristics is reduced, in a thin film transistor.

1. A thin film transistor comprising: an island-shaped semiconductorfilm and an island-shaped gate insulating film patterned by using thesame photomask over an insulating substrate; a side wall made of aninsulating material formed on a side face of the island-shapedsemiconductor film; and a gate electrode formed over the island-shapedgate insulating film, characterized in that the gate electrode overlapsthe side face of the island-shaped semiconductor film with the side walltherebetween.
 2. A thin film transistor comprising: an island-shapedsemiconductor film and an island-shaped gate insulating film patternedby using the same photomask over an insulating substrate; a side wallmade of an insulating material formed on side faces of the island-shapedsemiconductor film and the island-shaped gate insulating film; and agate electrode formed over the island-shaped gate insulating film,characterized in that the gate electrode overlaps the side face of theisland-shaped semiconductor film with the side wall therebetween.
 3. Athin film transistor comprising: an island-shaped semiconductor film andan island-shaped gate insulating film patterned by using the samephotomask on an insulating surface; and a gate electrode formed over theisland-shaped gate insulating film, charcterized in that a side face ofthe island-shaped semiconductor film is insulated, and the gateelectrode overlaps with the insulated side face of the island-shapedsemiconductor film.
 4. A thin film transistor comprising: anisland-shaped semiconductor film and an island-shaped gate insulatingfilm patterned by using the same photomask on an insulating substrate;an insulating film patterned to cover side faces of the island-shapedsemiconductor film and the island-shaped gate insulating film and only aperipheral portion of a top face of the island-shaped gate insulatingfilm; and a gate electrode formed over the island-shaped gate insulatingfilm, characterized in that the gate electrode overlaps the side face ofthe island-shaped semiconductor film with the insulating film patternedto cover the side faces of the island-shaped semiconductor film and theisland-shaped gate insulating film and only the peripheral portion ofthe top face of the island-shaped gate insulating film therebetween. 5.A thin film transistor according to claim 1 or 2, characterized in thateffective thickness of the side wall in a portion covering the side faceof the island-shaped semiconductor film in a direction perpendicular tothe side face is set equal to or thicker than effective thickness of theisland-shaped gate insulating film.
 6. A thin film transistor accordingto claim 3, characterized by regarding effective thickness of aninsulated portion of the side face of the island-shaped semiconductorfilm in a direction perpendicular to the side face as being set equal toor thicker than effective thickness of the island-shaped gate insulatingfilm.
 7. A thin film transistor according to claim 4, characterized byregarding effective thickness of the insulating film patterned to coverthe side faces of the island-shaped semiconductor film and theisland-shaped gate insulating film and only the peripheral portion ofthe top face of the island-shaped gate insulating film as being setequal to or thicker than effective thickness of the island-shaped gateinsulating film.
 8. A method for manufacturing a thin film transistor,characterized by comprising: forming a semiconductor film over aninsulating substrate; forming a first insulating film over thesemiconductor film; heat-treating the semiconductor film and the firstinsulating film; patterning the semiconductor film and the firstinsulating film into island shapes with the use of the same photomaskafter the heat treatment to form an island-shaped semiconductor film andan island-shaped gate insulating film; forming a second insulating filmover the island-shaped gate insulating film; etching the secondinsulating film anisotropically to form a side wall covering side facesof the island-shaped semiconductor film and the island-shaped gateinsulating film in self-aligned manner; forming a conductive film overthe island-shaped gate insulating film after forming the side wall; andpatterning the conductive film to form a gate electrode.
 9. A method formanufacturing a thin film transistor, characterized by comprising:forming a semiconductor film over an insulating substrate; forming aninsulating film over the semiconductor film; heat-treating thesemiconductor film and the insulating film; patterning the semiconductorfilm and the insulating film into island shapes with the use of oneresist mask after the heat treatment to form an island-shapedsemiconductor film and an island-shaped gate insulating film; insulatinga side face of the semiconductor film by adding oxygen or nitrogen to aside face of the island-shaped semiconductor film without removing theresist mask; forming a conductive film over the island-shaped gateinsulating film; and patterning the conductive film to form a gateelectrode.
 10. A method for manufacturing a thin film transistor,characterized by comprising: forming a semiconductor film over aninsulating substrate; forming a first insulating film over thesemiconductor film; heat-treating the semiconductor film and the firstinsulating film; patterning the semiconductor film and the firstinsulating film into island shapes with the use of the same photomaskafter the heat treatment to form an island-shaped semiconductor film andan island-shaped gate insulating film; forming a second insulating filmover the island-shaped gate insulating film; patterning the secondinsulating film to cover edge portions of the island-shapedsemiconductor film and the island-shaped gate insulating film and only aperipheral portion of a top face of the island-shaped gate insulatingfilm; forming a conductive film over the island-shaped gate insulatingfilm; and patterning the conductive film to form a gate electrode.
 11. Amethod for manufacturing a thin film transistor, characterized bycomprising: forming a semiconductor film over an insulating substrate;forming a first insulating film over the semiconductor film; forming afirst conductive film over the first insulating film; heat-treating thesemiconductor film, the first insulating film, and the first conductivefilm, patterning the semiconductor film, the first insulating film, andthe first conductive film into island shapes with the use of the samephotomask after the heat treatment to form an island-shapedsemiconductor film, an island-shaped gate insulating film, and a firstisland-shaped conductive film; forming a second insulating film over thefirst island-shaped conductive film; etching the second insulating filmanisotropically to form a side wall covering side faces of theisland-shaped semiconductor film, the island-shaped gate insulatingfilm, and the first island-shaped conductive film in a self-alignedmanner; forming a second conductive film over the first island-shapedconductive film after forming the side wall; and patterning the firstisland-shaped conductive film and the second conductive film to form agate electrode.
 12. A method for manufacturing a thin film transistor,characterized by comprising: forming a semiconductor film over aninsulating substrate; forming an insulating film over the semiconductorfilm; forming a first conductive film over the insulating film;heat-treating the semiconductor film, the insulating film, and the firstconductive film; patterning the semiconductor film, the insulating film,and the first conductive film into island shapes with the use of thesame resist mask after the heat treatment to form an island-shapedsemiconductor film, an island-shaped gate insulating film, and a firstisland-shaped conductive film; adding oxygen or nitrogen to a side faceof the island-shaped semiconductor film without removing the resist maskto insulate a side face of the semiconductor film; forming a secondconductive film over the first island-shaped conductive film; andpatterning the first island-shaped conductive film and the secondconductive film to form a gate electrode.
 13. A method for manufacturinga thin film transistor, characterized by comprising: forming asemiconductor film over an insulating substrate; forming a firstinsulating film over the semiconductor film; forming a first conductivefilm over the insulating film; heat-treating the semiconductor film, thefirst insulating film, and the first conductive film; patterning thesemiconductor film, the first insulating film, and the first conductivefilm into island shapes with the use of the same photomask after theheat treatment to form an island-shaped semiconductor film, anisland-shaped gate insulating film, and a first island-shaped conductivefilm; forming a second insulating film over the first island-shapedconductive film; patterning the second insulating film to cover edgeportions of the island-shaped semiconductor film, the island-shaped gateinsulating film, and the first island-shaped conductive film and only aperipheral portion of a top face of the first island-shaped conductivefilm; forming a second conductive film over the island-shaped gateinsulating film; and forming a gate electrode by patterning the firstconductive film and the second conductive film.
 14. A method formanufacturing a thin film transistor according to any one of claims 8,10, 11 and 13, characterized in that the heat-treatment of thesemiconductor film and the first insulating film is done at atemperature of from 600° C. to 800° C.
 15. A method for manufacturing athin film transistor according to claim 9 or 12, characterized in thatthe heat-treatment of the semiconductor film and the insulating film isdone at a temperature of from 600° C. to 800° C.
 16. A method formanufacturing a thin film transistor according to claim 14,characterized in that a strain point of the insulating substrate isequal to or lower than 600° C.
 17. A method for manufacturing a thinfilm transistor according to claim 15, characterized in that a strainpoint of the insulating substrate is equal to or lower than 600° C. 18.A method for manufacturing a thin film transistor according to claim 9,characterized in that the gate electrode is led outside theisland-shaped semiconductor film.
 19. A method for manufacturing a thinfilm transistor according to claim 10, characterized in that the gateelectrode is led outside the island-shaped semiconductor film.
 20. Amethod for manufacturing a thin film transistor according to claim 11,characterized in that the gate electrode is led outside theisland-shaped semiconductor film.
 21. A method for manufacturing a thinfilm transistor according to claim 12, characterized in that the gateelectrode is led outside the island-shaped semiconductor film.
 22. Amethod for manufacturing a thin film transistor according to claim 13,characterized in that the gate electrode is led outside theisland-shaped semiconductor film.
 23. An electronic apparatus comprisingthe thin film transistor according to any one of claim 1 to 4,characterized in that the electronic apparatus is selected from thegroup consisting of a light emitting device, a digital still camera, apersonal computer, a mobile computer, an image reproducing device, agoggle type display, a video camera, and a cellular phone.